A radiation detection element detects radiation and is used in fields of nuclear medicine, nuclear power, astronomy, cosmic-ray physics, and the like, for example. Here, radiation includes an alpha ray, a beta ray, a gamma ray, an X-ray, a neutron beam, a charged particle beam, and the like.
A conventional pixel type radiation detector employs such connection configuration that a sensor portion which performs photoelectric conversion and a read circuit portion which reads a signal charge from this sensor portion are produced as individual chips and electrical connection between respective chips is performed by electrical connection using a metal bump.
However, in order to electrically connect these two chips, joining with excessively large number of metal bumps is required, so that the thickness is increased and a mounting cost is also increased.
In PTL 1, a conventional CMOS image sensor is proposed. In this conventional CMOS image sensor, the silicon-on-insulator (SOI) structure which has a laminated structure of a single-crystal silicon layer/an insulator layer/a thick single-crystal silicon layer in which a thin single-crystal silicon layer is formed on a silicon substrate with an insulator layer interposed therebetween is employed.
On this conventional CMOS image sensor, a photodiode which is formed on a P-type silicon substrate from which an embedded oxide layer is removed and a read circuit which includes an amplifying transistor which reads and amplifies signal charges which are photoelectrically converted in the photodiode and accumulated are mounted.
If a radiation detection element and a read circuit can be integrated by using a technique in which an electronic circuit is constructed on one SOI substrate, in which an insulation silicon oxide film (an insulator layer) is formed on a silicon substrate and a silicon layer is further provided on the insulation silicon oxide film as PTL 1, enhancement of an S/N ratio brought by reduction of metal bumps and parasitic capacitance of wirings and increase in a reading speed brought by shortening of the wiring length can be expected.
However, such SOI substrate has such problem that threshold value variation caused by variation in film thicknesses of the SOI layer is generated. A read circuit is composed of an analog circuit, so that if there is variation in threshold values in a MOS transistor, a problem of characteristic variation arises.
Therefore, in PTL 2, such description is provided that an insulator layer provided under a gate electrode to be opposed to a channel region of a MOS transistor is formed to have a laminated structure composed of a silicon oxide film and a silicon nitride film and charges are accumulated in this laminated structure portion so as to adjust threshold value variation caused by film-thickness variation of an SOI layer.